FIG. 1 is a block diagram showing the structure of a conventional substrate potential adjusting apparatus, in particular an apparatus having a limiter. As shown in FIG. 1, a pump circuit 2 draws a current I.sub.BB from a semiconductor substrate 4 formed with a desired circuit thereon to thereby set the substrate to a certain voltage (substrate voltage) V.sub.BB. The pump circuit 2 is supplied with a periodical signal from a ring oscillator 1 which starts or stops oscillation in response to a control signal SLMT from a limiter circuit 3. A leak current I.sub.leak usually flows through the substrate 4 from the limiter circuit 3.
The operation of the substrate potential adjusting apparatus constructed as above will be described with reference to a timing chart shown in FIG. 2. This timing chart shows the substrate voltage V.sub.BB changing with time.
The limiter circuit 3 always monitors the substrate voltage V.sub.BB. When the substrate voltage V.sub.BB rises, the limiter circuit supplies the control signal SLMT to the ring oscillator 1. The ring oscillator 1 then starts operating and supplies a periodic signal to the pump circuit 2, so that the substrate 4 is set to the voltage V.sub.BB. This operation corresponds to the period I in FIG. 2. After the substrate voltage V.sub.BB is lowered, the limiter circuit 3 performs a limiter operation. Specifically, the control signal SLMT is inverted and the ring oscillator 1 stops its operation. As a result, the pump circuit 2 stops and the current I.sub.BB flows into the substrate 4 from the limiter circuit 3 to raise the substrate voltage V.sub.BB. This operation corresponds to the period II in FIG. 2. During the period III inclusive of the period I and the period II, the leak current I.sub.leak always flows through the substrate 4 from the limiter circuit 3. Since I.sub.BB I.sub.leak, the voltage V.sub.BB can be established by drawing the current I.sub.BB.
As described above, during the period I, the ring oscillator 1 and pump circuit 2 operate in response to the control signal SLMT from the limiter circuit 3. Therefore, a large current is consumed. When the substrate 4 is set to the voltage V.sub.BB by the current I.sub.BB, the limiter circuit 3 detects it. At this state, the period II starts. During the period II, the control signal SLMT from the limiter circuit 3 is inverted to stop operating the ring oscillator 1 and pump circuit 2. Therefore, only the leak current I.sub.leak flows through the substrate 4, reducing an average current.
In other words, the period I is the operation period of the ring oscillator 1 during which the current I.sub.BB is drawn from the substrate. The period II is the operation stop period of the ring oscillator 1 and pump circuit 2 during which I.sub.BB is zero. The period III is the period during which the current I.sub.leak flows through the substrate 4 from the limiter circuit 3.
The relation between the substrate voltage V.sub.BB and power supply voltage V.sub.DD is shown in FIG. 3A. In FIG. 3A, .alpha. represents the characteristic of the pump circuit 2 assuming that the ring oscillator 1 operates always, and .beta. represents the characteristic of the limiter circuit 3. The relation of total consumption current I.sub.DD and power supply voltage V.sub.DD is shown in FIG. 3B.
In FIGS. 3A and 3B, in the region A, a potential set by the pump circuit 2 is shallower than a potential set by the limiter circuit 3. Therefore, in this region A, the ring oscillator 1 and pump circuit 2 always operate, and the consumption current I.sub.DD increases greatly as the power supply voltage V.sub.DD becomes high. In the region B, a potential set by the pump circuit 2 is deeper than a potential set by the limiter circuit 3. Therefore, in this region B, under control of the limiter circuit 3, the substrate voltage V.sub.BB remains at the substrate voltage V.sub.BB determined by the characteristic of the limiter circuit 3, and the ring oscillator 1 and pump circuit 2 operate intermittently, reducing the total consumption current. Namely, in the region A, the ring oscillator 1 and pump circuit 2 always operate, and in the region B, the ring oscillator 1 and pump circuit 2 operate intermittently under control of the limiter circuit 3. A peak current I.sub.DDpeak is therefore present at a peak power supply voltage V.sub.DDP at the boundary between the regions A and B. The peak power supply voltage V.sub.DDP is determined as an intersection between the characteristic lines of the pump circuit 2 and limiter circuit 3.
A conventional substrate potential adjusting apparatus has been structured in the manner described above. With such an apparatus, the peak power supply voltage V.sub.DDP (at which the peak current I.sub.DDpeak flows) is smaller than 4.5 V within the power supply voltage V.sub.DD range from 4.5 V to 5.5 V. Therefore, a peak consumption current will not appear within the rated power supply voltage range, posing no practical problem. If low power consumption such as during battery backup is to be expected, it is necessary to consider a low voltage operation. In this case, a presence of a peak consumption current should be taken into consideration. For example, consider the case where an operation voltage is set about 3V near the peak power supply voltage V.sub.DDP. This case is associated with the problem of the peak consumption current, so that it is difficult to realize such low power consumption. It is conceivable to provide a countermeasure against this by setting the voltage during a low voltage operation slightly higher than the peak power supply voltage V.sub.DDP. With this setting, however, a battery is rapidly consumed by the peak current as the supply voltage of the battery lowers.